Newton's Law of Gravity - Basic Principles Newton's Law of Gravity Sign Up for Our Free Newsletters Thanks, You're in! What I Learned About Today You might also enjoy: Health Tip of the Day Recipe of the Day There was an error. Please try again. Please select a newsletter. Please enter a valid email address. Did you mean ? Newton did think about gravity watching apples fall from trees, but he didn't have a "eureka" moment by having on fall on his head. That's just not true!. pinstock, Getty Images Updated August 10, 2016. Newton's law of gravity defines the attractive force between all objects that possess mass . Understanding the law of gravity, one of the fundamental forces of physics , offers profound insights into the way our universe functions. The Proverbial Apple The famous story that Isaac Newton came up with the idea for the law of gravity by having an apple fall on his head is not true, although he did begin thinking about the issue on his mother's farm when he saw an apple fall from a tree. He wondered if the same force at work on the apple was also at work on the moon. If so, why did the apple fall to the Earth and not the moon?

Along with his Three Laws of Motion , Newton also outlined his law of gravity in the 1687 book Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy) , which is generally referred to as the Principia. continue reading below our video What Are the Major Laws of Physics? Johannes Kepler (German physicist, 1571-1630) had developed three laws governing the motion of the five then-known planets. He did not have a theoretical model for the principles governing this movement, but rather achieved them through trial and error over the course of his studies. Newton's work, nearly a century later, was to take the laws of motion he had developed and apply them to planetary motion to develop a rigorous mathematical framework for this planetary motion. Gravitational Forces Newton eventually came to the conclusion that, in fact, the apple and the moon were influenced by the same force.

He named that force gravitation (or gravity) after the Latin word gravitas which literally translates into "heaviness" or "weight." In the Principia, Newton defined the force of gravity in the following way (translated from the Latin): Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them. Mathematically, this translates into the force equation: FG = Gm1m2/r2 In this equation, the quantities are defined as: Fg = The force of gravity (typically in newtons) G = The gravitational constant, which adds the proper level of proportionality to the equation. The value of G is 6.67259 x 10-11 N * m2 / kg2, although the value will change if other units are being used. m1 & m1 = The masses of the two particles (typically in kilograms) r = The straight-line distance between the two particles (typically in meters) Interpreting the Equation This equation gives us the magnitude of the force, which is an attractive force and therefore always directed toward the other particle.

As per Newton's Third Law of Motion , this force is always equal and opposite. Newton's Three Laws of Motion give us the tools to interpret the motion caused by the force and we see that the particle with less mass (which may or may not be the smaller particle, depending upon their densities) will accelerate more than the other particle. This is why light objects fall to the Earth considerably faster than the Earth falls toward them. Still, the force acting on the light object and the Earth is of identical magnitude, even though it doesn't look that way. It is also significant to note that the force is inversely proportional to the square of the distance between the objects. As objects get further apart, the force of gravity drops very quickly. At most distances, only objects with very high masses such as planets, stars, galaxies, and black holes have any significant gravity effects. Center of Gravity In an object composed of many particles , every particle interacts with every particle of the other object. Since we know that forces ( including gravity ) are vector quantities , we can view these forces as having components in the parallel and perpendicular directions of the two objects.

In some objects, such as spheres of uniform density, the perpendicular components of force will cancel each other out, so we can treat the objects as if they were point particles, concerning ourselves with only the net force between them. The center of gravity of an object (which is generally identical to its center of mass) is useful in these situations. We view gravity, and perform calculations, as if the entire mass of the object were focused at the center of gravity. In simple shapes - spheres, circular disks, rectangular plates, cubes, etc. - this point is at the geometric center of the object. This idealized model of gravitational interaction can be applied in most practical applications, although in some more esoteric situations such as a non-uniform gravitational field, further care may be necessary for the sake of precision. Gravity Index Who Discovered Gravity? - Universe Today Universe Today by Matt Williams Four fundamental forces govern all interactions within the Universe. They are weak nuclear forces, strong nuclear forces, electromagnetism, and gravity. Of these, gravity is perhaps the most mysterious.

While it has been understood for some time how this law of physics operates on the macro-scale – governing our Solar System, galaxies, and superclusters – how it interacts with the three other fundamental forces remains a mystery. Naturally, human beings have had a basic understanding of this force since time immemorial. And when it comes to our modern understanding of gravity, credit is owed to one man who deciphered its properties and how it governs all things great and small – Sir Isaac Newton . Thanks to this 17th century English physicist and mathematician, our understanding of the Universe and the laws that govern it would forever be changed. While we are all familiar with the iconic image of a man sitting beneath an apple tree and having one fall on his head, Newton’s theories on gravity also represented a culmination of years worth of research, which in turn was based on centuries of accumulated knowledge. He would present these theories in his magnum opus, Philosophiae Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”), which was first published in 1687.

In this volume, Newton laid out what would come to be known as his Three Laws of Motion , which were derived from Johannes Kepler ’s Laws of Planetary Motion and his own mathematical description of gravity. These laws would lay the foundation of classical mechanics, and would remain unchallenged for centuries – until the 20th century and the emergence of Einstein’s Theory of Relativity . Newton’s own copy of his Principia, with hand-written corrections for the second edition. Credit: Trinity Cambridge/Andrew Dunn Physics by 17th Century: The 17th century was a very auspicious time for the sciences, with major breakthroughs occurring in the fields of mathematics, physics, astronomy, biology and chemistry. Some of the greatest developments in the period include the development of the heliocentric model of the Solar System by Nicolaus Copernicus , the pioneering work with telescopes and observational astronomy by Galileo Galilei , and the development of modern optics. It was also during this period that Johannes Kepler developed his Laws of Planetary Motion.

Formulated between 1609 and 1619, these laws described the motion of the then-known planets ( Mercury , Venus , Earth , Mars , Jupiter , and Saturn ) around the Sun. They stated that: Planets move around the Sun in ellipses, with the Sun at one focus The line connecting the Sun to a planet sweeps equal areas in equal times. The square of the orbital period of a planet is proportional to the cube (3rd power) of the mean distance from the Sun in (or in other words–of the”semi-major axis” of the ellipse, half the sum of smallest and greatest distance from the Sun). These laws resolved the remaining mathematical issues raised by Copernicus’ heliocentric model, thus removing all doubt that it was the correct model of the Universe. Working from these, Sir Isaac Newton began considering gravitation and its effect on the orbits of planets. A comparison of the geocentric and heliocentric models of the universe. Credit: history.ucsb.edu Newton’s Three Laws: In 1678, Newton suffered a complete nervous breakdown due to overwork and a feud with fellow astronomer Robert Hooke.

For the next few years, he withdrew from correspondence with other scientists, except where they initiated it, and renewed his interest in mechanics and astronomy. In the winter of 1680-81, the appearance of a comet, about which he corresponded with John Flamsteed (England’s Astronomer Royal) also renewed his interest in astronomy. After reviewing Kepler’s Laws of Motion, Newton developed a mathematical proof that the elliptical form of planetary orbits would result from a centripetal force inversely proportional to the square of the radius vector. Newton communicated these results to Edmond Halley (discoverer of “Haley’s Comet”) and to the Royal Society in his De motu corporum in gyrum. This tract, published in 1684, contained the seed of what Newton would expand to form his magnum opus, the Philosophiae Naturalis Principia Mathematica. This treatise, which was published in July of 1687, contained Newton’s three laws of motion, which stated that: When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.

The vector sum of the external forces (F) on an object is equal to the mass (m) of that object multiplied by the acceleration vector (a) of the object. In mathematical form, this is expressed as: F=ma When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body. Together, these laws described the relationship between any object, the forces acting upon it and the resulting motion, laying the foundation for classical mechanics. The laws also allowed Newton to calculate the mass of each planet, the flattening of the Earth at the poles, and the bulge at the equator, and how the gravitational pull of the Sun and Moon create the Earth’s tides. In the same work, Newton presented a calculus-like method of geometrical analysis using ‘first and last ratios’, worked out the speed of sound in air (based on Boyle’s Law ), accounted for the procession of the equinoxes (which he showed were a result of the Moon’s gravitational attraction to the Earth), initiated the gravitational study of the irregularities in the motion of the moon, provided a theory for the determination of the orbits of comets, and much more.

Newton and the “Apple Incident”: The story of Newton coming up with his theory of universal gravitation as a result of an apple falling on his head has become a staple of popular culture. And while it has often been argued that the story is apocryphal and Newton did not devise his theory at any one moment, Newton himself told the story many times and claimed that the incident had inspired him. In addition, the writing’s of William Stukeley – an English clergyman, antiquarian and fellow member of the Royal Society – have confirmed the story. But rather than the comical representation of the apple striking Newton on the head, Stukeley described in his Memoirs of Sir Isaac Newton’s Life (1752) a conversation in which Newton described pondering the nature of gravity while watching an apple fall. “…we went into the garden, & drank thea under the shade of some appletrees; only he, & my self. amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind.

“why should that apple always descend perpendicularly to the ground,” thought he to himself; occasion’d by the fall of an apple…” John Conduitt, Newton’s assistant at the Royal Mint (who eventually married his niece), also described hearing the story in his own account of Newton’s life. According to Conduitt, the incident took place in 1666 when Newton was traveling to meet his mother in Lincolnshire. While meandering in the garden, he contemplated how gravity’s influence extended far beyond Earth, responsible for the falling of apple as well as the Moon’s orbit. Similarly, Voltaire wrote n his Essay on Epic Poetry (1727) that Newton had first thought of the system of gravitation while walking in his garden and watching an apple fall from a tree. This is consistent with Newton’s notes from the 1660s, which show that he was grappling with the idea of how terrestrial gravity extends, in an inverse-square proportion, to the Moon. Sapling of the reputed original tree that inspired Sir Isaac Newton to consider gravitation.

Credit: Wikipedia Commons/Loodog However, it would take him two more decades to fully develop his theories to the point that he was able to offer mathematical proofs, as demonstrated in the Principia. Once that was complete, he deduced that the same force that makes an object fall to the ground was responsible for other orbital motions. Hence, he named it “universal gravitation”. Various trees are claimed to be “the” apple tree which Newton describes. The King’s School, Grantham , claims their school purchased the original tree, uprooted it, and transported it to the headmaster’s garden some years later. However, the National Trust, which holds the Woolsthorpe Manor (where Newton grew up) in trust, claims that the tree still resides in their garden. A descendant of the original tree can be seen growing outside the main gate of Trinity College, Cambridge, below the room Newton lived in when he studied there. Newton’s work would have a profound effect on the sciences, with its principles remaining canon for the following 200 years.

It also informed the concept of universal gravitation, which became the mainstay of modern astronomy, and would not be revised until the 20th century – with the discovery of quantum mechanics and Einstein’s theory of General Relativity . Astronomy Cast has some two good episodes on the subject. Here’s Episode 37: Gravitational Lensing , and Episode 102: Gravity , Sources: Gravity The History of Gravity Humans only recently (like in the last 300 years) realized what Gravity is all about. Greek philosophers thought that the planets and stars were part of the gods' realm and followed a "natural motion." They did not realize that Gravity is involved. The Greeks' ideas stuck around until the 16th century. Beginning in the 1500s, though, astronomers like Galileo and Brahe discovered that the earth and other planets revolved around the sun. Kepler showed that they moved in an eliptical orbit, not a circle. The question was why. Sir Isaac Newton -- The Discoverer of Gravity! Sir Isaac Newton was an English mathematician and mathematician and physicist who lived from 1642-1727.

The legend is that Newton discovered Gravity when he saw a falling apple while thinking about the forces of nature. Whatever really happened, Newton realized that some force must be acting on falling objects like apples because otherwise they would not start moving from rest. Newton also realized that the moon would fly off away from Earth in a straight line tangent to its orbit if some force was not causing it to fall toward the Earth. The moon is only a projectile circling around the Earth under the attraction of Gravity. Newton called this force "gravity" and determined that gravitational forces exist between all objects. Using the idea of Gravity, Newton was able to explain the astronomical observations of Kepler. The work of Galileo, Brahe, Kepler, and Newton proved once and for all that the Earth wasn't the center of the solar system. The Earth, along with all other planets,orbits around the sun. Two astronomers, J.C. Adams and U.J.J. LeVerrier, later used the concept of Gravity to predict that the planet Neptune would be discovered. They realized that there must be another planet exerting a gravitational force on Uranus because Uranus had odd perturbations in its orbit.

(Perturbations are deviations in orbits.) Albert Einstein -- The General Theory of Relativity Einstein developed a whole new idea about Gravity. According to Einstein, Gravity arises from the "warping" of space and time. Einstein's new theory of Gravity explains a number of phenomena that would violate Newton's theory. For example, light bends when passing near massive objects like the Sun. And a clock raised above the Earth speeds up relative to a clock on the surface. Basic Facts About Gravity Gravity is a force of attraction that exists between any two objects. There is a force of gravity between the sun and the Earth, between the Earth and us, and even between two marbles. Projectiles, satellites, planets, galaxies, and clusters of galaxies are all influenced by Gravity. Gravity is the weakest of the four known forces of nature, yet the most dominant force. Even though it's the weakest force, Gravity holds together entire solar systems and galaxies! The law of universal gravitation says that every object attracts every other object with a force that, for any two objects, is directly proportional to the mass of each object and inversely proportional to the square of the distance between the two objects.

The gravitational formula is This is an example of the "inverse-square law": gravitational force varies as the inverse square of the distance between the two objects. As a result, the effect of Gravity falls rapidly as the distance increases between two objects. The best current estimate of G is that it eqauls about 6.67259 10-11 newton-square meter per square kilogram. The gravitational field for a planet, g, equals GM/Rsquared, where G is the mass of the planet and R is the distance of the object from the center of the planet (the planet's radius if the object is on the surface of the planet). This means that Gravity is greater where a planet is more massive and where it has a smaller radius. So even though Mars has only about 1/10th of the mass of the Earth, the gravitational force on the surface of Mars is more than 1/10th that on the surface of the Earth -- because Mars' surface is closer to the planet's surface! The force of attraction between you and the Earth is your weight. Gravity determines the "escape speed" for an object like a rocket. The stronger the gravitational pull of the object, the larger the escape speed.

The following chart shows the escape speed for the sun, two planets, and the Earth's moon. Sun Law of Gravity Sir Isaac Newton The Universal Law of Gravitation There is a popular story that Newton was sitting under an apple tree, an apple fell on his head, and he suddenly thought of the Universal Law of Gravitation. As in all such legends, this is almost certainly not true in its details, but the story contains elements of what actually happened. What Really Happened with the Apple? Probably the more correct version of the story is that Newton, upon observing an apple fall from a tree, began to think along the following lines: The apple is accelerated, since its velocity changes from zero as it is hanging on the tree and moves toward the ground. Thus, by Newton's 2nd Law there must be a force that acts on the apple to cause this acceleration. Let's call this force "gravity", and the associated acceleration the "acceleration due to gravity". Then imagine the apple tree is twice as high. Again, we expect the apple to be accelerated toward the ground, so this suggests that this force that we call gravity reaches to the top of the tallest apple tree.

Sir Isaac's Most Excellent Idea Now came Newton's truly brilliant insight: if the force of gravity reaches to the top of the highest tree, might it not reach even further; in particular, might it not reach all the way to the orbit of the Moon! Then, the orbit of the Moon about the Earth could be a consequence of the gravitational force, because the acceleration due to gravity could change the velocity of the Moon in just such a way that it followed an orbit around the earth. Newton knew that the force which caused the apple's acceleration (gravity) must be dependent upon the mass of the apple. And since the force acting to cause the apple's downward acceleration also causes the earth's upward acceleration (Newton's third law), that force must also depend upon the mass of the earth. So for Newton, the force of gravity acting between the earth and any other object is directly proportional to the mass of the earth, directly proportional to the mass of the object, and inversely proportional to the square of the distance which separates the centers of the earth and the object. The constant of proportionality G is known as the universal gravitational constant.

It is termed a "universal constant" because it is thought to be the same at all places and all times, and thus universally characterizes the intrinsic strength of the gravitational force. The numerical value of G is very small, which is basically the reason for the force of gravity to be the weakest force of the nature. For the value of "G", refer to your text book.. But Newton's law of universal gravitation extends gravity beyond earth. Newton's law of universal gravitation is about the universality of gravity. Newton's place in the Gravity Hall of Fame is not due to his discovery of gravity, but rather due to his discovery that gravitation is universal. ALL objects attract each other with a force of gravitational attraction. Gravity is universal. This force of gravitational attraction is directly dependent upon the masses of both objects and inversely proportional to the square of the distance which separates their centers. Weight and the Gravitational Force We have seen that in the Universal Law of Gravitation the crucial quantity is mass. In popular language mass and weight are often used to mean the same thing; in reality they are related but quite different things. What we commonly call weight is really just the gravitational force exerted on an object of a certain mass.

We can illustrate by choosing the Earth as one of the two masses in the previous illustration of the Law of Gravitation: Thus, the weight of an object of mass m at the surface of the Earth is obtained by multiplying the mass m by the acceleration due to gravity, g, at the surface of the Earth. The acceleration due to gravity is approximately the product of the universal gravitational constant G and the mass of the Earth M, divided by the radius of the Earth, r, squared. (We assume the Earth to be spherical and neglect the radius of the object relative to the radius of the Earth in this discussion.) The measured gravitational acceleration at the Earth's surface is found to be about 980 cm/second/second. Mass and Weight Mass is a measure of how much material is in an object, but weight is a measure of the gravitational force exerted on that material in a gravitational field; thus, mass and weight are proportional to each other, with the acceleration due to gravity as the proportionality constant. It follows that mass is constant for an object (actually this is not quite true as described by the Relativity Theory), but weight depends on the location of the object.

For example, if we transported the preceding object of mass m to the surface of the Moon, the gravitational acceleration would change because the radius and mass of the Moon both differ from those of the Earth. Thus, our object has mass m both on the surface of the Earth and on the surface of the Moon, but it will weigh much less on the surface of the Moon because the gravitational acceleration there is a factor of 6 less than at the surface of the Earth. Using Equations as a Guide to Thinking The inverse square law proposed by Newton suggests that the force of gravity acting between any two objects is inversely proportional to the square of the separation distance between the object's centers. Altering the separation distance (r) results in an alteration in the force of gravity acting between the objects. Since the two quantities are inversely proportional, an increase in one quantity results in a decrease in the value of the other quantity. That is, an increase in the separation distance causes a decrease in the force of gravity and a decrease in the separation distance causes an increase in the force of gravity. Furthermore, the factor by which the force of gravity is changed is the square of the factor by which the separation distance is changed.

So if the separation distance is doubled (increased by a factor of 2), then the force of gravity is decreased by a factor of four (2 raised to the second power). And if the separation distance (r) is tripled (increased by a factor of 3), then the force of gravity is decreased by a factor of nine (3 raised to the second power). Thinking of the force-distance relationship in this way involves using a mathematical relationship as a guide to thinking about how an alteration in one variable effects the other variable. Equations can be more than merely recipes for algebraic problem-solving; they can be "guides to thinking." The proportionalities expressed by Newton's universal law of gravitation is represented graphically by the following illustration. Observe how the force of gravity is directly proportional to the product of the two masses and inversely proportional to the square of the distance of separation. In the above figure, the figure on the left hand side indicates the effect of "mass" if the diatnce between the two objects remains fixed at a given value "d". The right hand figure shows the effect of changing the distance while keeping the mass constant, and the last part of it shows the effect of changing both the distance and the mass.

Check your understanding of the inverse square law as a guide to thinking by answering the following questions below. Check Your Understanding 1 . Suppose that two objects attract each other with a force of 16 units (like 16 N or 16 lb). If the distance between the two objects is doubled, what is the new force of attraction between the two objects? Answer: If the distance is increased by a factor of 2, then distance squared will increase by a factor of 4. Thus, the inverse square law implies that the force will be "1/4" of the original 16 units. Therefore, the force of gravity becomes 4 units. 2. Suppose the distance in question 1 is tripled. What happens to the forces between the two objects? Answer: Again using inverse square law, we get distance squared to go up by a factor of 9. The force decreases by a factor of 9 and becomes 1.78 units. 3. If you wanted to make a profit in buying gold by weight at one altitude and selling it at another altitude for the same price per weight, should you buy or sell at the higher altitude location? What kind of scale must you use for this work?

Answer: To profit, buy at a high altitude and sell at a low altitude. Explanation is left to the student. Check Your Understanding 4. What would happen to your weight if the mass of the Earth somehow increased by 10%? Answer: Your weight is nothing but force of gravity between the earth and you (as an object with a mass m). As shown in the above graph, changing one of the masses results in change in force of gravity. In this case, if the earth' mass goes up by 10%, then the force of gravity on you, or your weight, will increase by the same amount, that is 10%. 5. The planet Jupiter is more than 300 times as massive as Earth, so it might seem that a body on the surface of Jupiter would weigh 300 times as much as on Earth. But it so happens a body would scarcely weigh three times as much on the surface of Jupiter as it would on the surface of the Earth. Explain why this is so. Answer: The effect of greater mass of Jupiter is partly off set by its larger radius which is about 10 times the radius of the earth.

This means the object is times farther from the center of the Jupiter compared to the earth. Inverse of the distance brings a factor of 100 to the denominator and as a result, the force increases by a factor of 300 due to the mass, but decreases by a factor of 100 due to the distance squared. The net effect is that the force increases 3 times. Planetary and Satellite Motion Biography: Sir Isaac Newton Biography: Sir Isaac Newton By Mary Bellis Updated August 14, 2016. Isaac Newton was born in 1642 in a manor house in Lincolnshire, England. His father had died two months before his birth. When Newton was three his mother remarried and he remained with his grandmother. He was not interested in the family farm so he was sent to Cambridge University to study. Isaac was born just a short time after the death of Galileo , one of the greatest scientists of all time. Galileo had proved that the planets revolve around the sun, not the earth as people thought at the time. Isaac Newton was very interested in the discoveries of Galileo and others.

Isaac thought the universe worked like a machine and that a few simple laws governed it. Like Galileo, he realized that mathematics was the way to explain and prove those laws. He formulated laws of motion and gravitation. These laws are math formulas that explain how objects move when a force acts on them. Isaac published his most famous book, Principia in 1687 while he was a mathematics professor at Trinity College in Cambridge. continue reading below our video Profile of Isaac Newton In the Principia, Isaac explained three basic laws that govern the way objects move. He also described his theory of gravity, the force that causes things to fall down. Newton then used his laws to show that the planets revolve around the suns in orbits that are oval, not round. The three laws are often called Newton’s Laws. The first law states that an object that is not being pushed or pulled by some force will stay still or will keep moving in a straight line at a steady speed. For example, if someone is riding a bike and jumps off before the bike is stopped what happens? The bike continues on until it falls over. The tendency of an object to remain still or keep moving in a straight line at a steady speed is called inertia.

The Second Law explains how a force acts on an object. An object accelerates in the direction the force is moving it. If someone gets on a bike and pushes the pedals forward the bike will begin to move. If someone gives the bike a push from behind, the bike will speed up. If the rider pushes back on the pedals the bike will slow down. If the rider turns the handlebars, the bike will change direction. The Third Law states that if an object is pushed or pulled, it will push or pull equally in the opposite direction. If someone lifts a heavy box, they use force to push it up. The box is heavy because it is producing an equal force downward on the lifter’s arms. The weight is transferred through the lifter’s legs to the floor. The floor also presses upward with an equal force. If the floor pushed back with less force, the person lifting the box would fall through the floor. If it pushed back with more force the lifter would fly up toward the air. When most people think of Isaac Newton, they think of him sitting under an apple tree observing an apple fall to the ground. When he saw the apple fall, Newton began to think about a specific kind of motion called gravity.

Newton understood that gravity was a force of attraction between two objects. He also understood that an object with more matter or mass exerted the greater force, or pulled smaller objects toward it. That meant that the large mass of the earth pulled objects toward it. That is why the apple fell down instead of up and why people don’t float in the air. He also thought that maybe gravity was not just limited to the earth and the objects on the earth. What if gravity extended to the moon and beyond? Newton calculated the force needed to keep the moon moving around the earth. Then he compared it with the force that made the apple fall downward. After allowing for the fact that the moon is much farther from the earth, and has a much greater mass, he discovered that the forces were the same and that the moon is also held in orbit around earth by the pull of earth’s gravity. Newton’s calculations changed the way people understood the universe. Prior to Newton, no one had been able to explain why the planets stayed in their orbits. What held them in place? People had thought that the planets were held in place by an invisible shield.

Isaac proved that they were held in place by the sun’s gravity and that the force of gravity was affected by distance and by mass. While he was not the first to understand that the orbit of a planet was elongated like an oval, he was the first to explain how it worked. Newton's theory of "Universal Gravitation" 22c. Flight (1) The Moon orbits around the Earth. Since its size does not appear to change, its distance stays about the same, and hence its orbit must be close to a circle. To keep the Moon moving in that circle--rather than wandering off--the Earth must exert a pull on the Moon, and Newton named that pulling force gravity. Was that the same force which pulled all falling objects downward? Supposedly, the above question occured to Newton when he saw an apple falling from a tree. John Conduitt, Newton's assistant at the royal mint and husband of Newton's niece, had this to say about the event when he wrote about Newton's life: In the year 1666 he retired again from Cambridge ...

to his mother in Lincolnshire & while he was musing in a garden it came into his thought that the power of gravity (which brought an apple from a tree to the ground) was not limited to a certain distance from earth, but that this power must extend much further than was usually thought. Why not as high as the Moon thought he to himself & that if so, that must influence her motion & perhaps retain her in her orbit, whereupon he fell a-calculating what would be the effect of that superposition... ( Keesing, R.G., The History of Newton's apple tree, Contemporary Physics, 39, 377-91, 1998) If it was the same force, then a connection would exist between the way objects fell and the motion of the Moon around Earth, that is, its distance and orbital period. The orbital period we know--it is the lunar month, corrected for the motion of the Earth around the Sun, which also affects the length of time between one "new moon" and the next. The distance was first estimated in ancient Greece--see here and here .

To calculate the force of gravity on the Moon, one must also know how much weaker it was at the Moon's distance. Newton showed that if gravity at a distance R was proportional to 1/R2 (varied like the "inverse square of the distance"), then indeed the acceleration g measured at the Earth's surface would correctly predict the orbital period T of the Moon. Newton went further and proposed that gravity was a "universal" force, and that the Sun's gravity was what held planets in their orbits. He was then able to show that Kepler's laws were a natural consequence of the "inverse squares law" and today all calculations of the orbits of planets and satellites follow in his footsteps. Nowadays students who derive Kepler's laws from the "inverse-square law" use differential calculus, a mathematical tool in whose creation Newton had a large share. Interestingly, however, the proof which Newton published did not use calculus, but relied on intricate properties of ellipses and other conic sections. Richard Feynman, Nobel-prize winning maverick physicist, rederived such a proof (as have some distinguished predecessors); see reference at the end of the section.

Here we will retrace the calculation, which linked the gravity observed on Earth with the Moon's motion across the sky, two seemingly unrelated observations. If you want to check the calculation, a hand-held calculator is helpful. Calculating the Moon's Motion We assume that the Moon's orbit is a circle, and that the Earth's pull is always directed toward's the Earth's center. Let RE be the average radius of the Earth (first estimated by Erathosthenes) RE= 6 371 km The distance R to the Moon is then about 60 RE. If a mass m on Earth is pulled by a force mg, and if Newton's "inverse square law" holds, then the pull on the same mass at the Moon's distance would be 602 = 3600 times weaker and would equal mg/3600 If m is the mass of the Moon, that is the force which keeps the Moon in its orbit. If the Moon's orbit is a circle, since R = 60 RE its length is 2 π R = 120 π RE Suppose the time required for one orbit is T seconds.

The velocity v of the motion is then v = distance/time = 120 π RE/T (Please note: gravity is not what gives the Moon its velocity. Whatever velocity the Moon has was probably acquired when it was created. But gravity prevents the Moon from running away, and confines it to some orbit.) The centripetal force holding the Moon in its orbit must therefore equal mv2/R = mv2/(60 RE) Is Gravity a Theory or a Law? | The Happy Scientist Science Videos Is Gravity a Theory or a Law? I frequently get emails wanting to know whether gravity is a law or a theory. That question brings up so many more questions that I thought it would be fun to explore. To try this, you will need: - an object to drop. OK, pick an object that will not break, dent the floor, cause a mess, or get either of us in trouble. Hold it out in front of you and release it. What happens? It falls, of course (unless you picked a helium balloon. In that case, gravity causes it to float upwards, by pulling downwards with more force on the air around the balloon). The gravitational attraction between the Earth and the object pulls it towards the ground.

But, when we do this experiment, should we be talking about the Law of Gravity or the Theory of Gravity? Actually, we should be talking about both. To understand why, we need to understand the scientific meaning of the words "law" and "theory." In the language of science, the word "law" describes an analytic statement. It gives us a formula that tells us what things will do. For example, Newton's Law of Universal Gravitation tells us: "Every point mass attracts every single point mass by a force pointing along the line intersecting both points. The force is directly proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses." So if we know the mass of two objects, and the distance between the center of mass of the two objects, we can calculate the gravitational pull between the Earth and the object you dropped, between the Sun and Mars, or between me and a bowl of ice cream.

We can use Newton's Law of Universal Gravitation to calculate how strong the gravitational pull is between the Earth and the object you dropped, which would let us calculate its acceleration as it falls, how long it will take to hit the ground, how fast it would be going at impact, how much energy it will take to pick it up again, etc. While the law lets us calculate quite a bit about what happens, notice that it does not tell us anything about WHY it happens. That is what theories are for. In the language of science, the word "theory" is used to describe an explanation of why and how things happen. For gravity, we use Einstein's Theory of General Relativity to explain why things fall. A theory starts as one or more hypotheses, untested ideas about why something happens. For example, I might propose a hypothesis that the object that you released fell because it was pulled by the Earth's magnetic field. Once we started testing, it would not take long to find out that my hypothesis was not supported by the evidence. Non-magnetic objects fall at the same rate as magnetic objects. Because it was not supported by the evidence, my hypothesis does not gain the status of being a theory.

To become a scientific theory, an idea must be thoroughly tested, and must be an accurate and predictive description of the natural world. While laws rarely change, theories change frequently as new evidence is discovered. Instead of being discarded due to new evidence, theories are often revised to include the new evidence in their explanation. The Theory of General Relativity has adapted as new technologies and new evidence have expanded our view of the universe. So when we are scientifically discussing gravity, we can talk about the law of gravity that describes the attraction between two objects, and we can also talk about the theory of gravity that describes why the objects attract each other. Isaac Newton Biography | Facts, Quotes & Inventions Isaac Newton Biography March 24, 2016 07:37pm ET MORE Isaac Newton laid the blueprints for his three laws of motion, still recited by physics students, in 1666. Credit: Library of Congress Isaac Netwon is synonymous with apples and gravity. He rose to become the most influential scientist of the 17th century, his ideas becoming the foundation of modern physics, after very humble beginnings. But first, the big question: Did an apple really fall on Newton's head and spur him to figure out gravity?

Historians say there is likely no more than a grain of truth to the story. Sir Isaac Newton was born, premature and tiny, in 1642 in Woolsthorpe, England. His father, wealthy but uneducated, died before Newton was born, and he ended up being raised by his grandmother after his mother remarried. It’s said he didn’t excel at school, but he ended up studying law at Trinity College Cambridge, part of Cambridge University. He worked as a servant to pay his bills. And he kept a journal about his ideas. What got Newton interested in math? He bought a book on the subject and couldn't comprehend it. After getting his bachelor's degree in 1665; he studied math, physics, optics and astronomy on his own (Cambridge was closed for a couple of years due to the plague known as the Black Death). By 1666 he had completed his early work on his three laws of motion . Later he got his master's degree.

Later work focused on the diffraction of light (he used a prism to discover that white light is made of a spectrum of colors ) and the concepts he'd become known for: universal gravitation, centrifugal force, centripetal force, and the effects and characteristics of bodies in motion. His laws are still used by physics students today: An object will remain in a state of inertia unless acted upon by force. The relationship between acceleration and applied force is F=ma. For every action there is an equal and opposite reaction. Isaac Newton quotes Newton said many things worth remembering, including these philosophical gems: "I can calculate the motion of heavenly bodies, but not the madness of people." "To myself I am only a child playing on the beach, while vast oceans of truth lie undiscovered before me." Newton once said that if he had achieved anything in his research, it was "by standing on the shoulders of giants ." The quote was prophetic. A couple of centuries later, Albert Einstein puzzled over how to reconcile Newton's law of gravity with special relativity, which after several years led to Einstein's theory of general relativity .

Isaac Newton inventions While he's best known for his work on gravity, Newton was a tinkerer, too, but more with ideas than physical inventions. He did invent reflecting lenses for telescopes, which produced clearer images in a smaller telescope compared with the refracting models of the time. In his later years, he developed anti-counterfeiting measures for coins, including the ridges you see on quarters today. Among his biggest " inventions " was calculus. Yes, that's right. Mere math and algebra weren't enough to explain the ideas in his head, so he helped invent calculus (German mathematician Gottfried Leibniz is typically credited with developing it independently at about the same time). It's said that Newton invented a cat door so his cats would stop scratching to get in, but the truth of that one is a bit sketchy. He also conceived of an "orbital cannon" that would poke out of a huge mountain, up in space, and with just the right amount of gunpowder could put a cannonball into orbit. This was not something Newton actually imagined building, but rather a way to think about his theories.

Later years Urged by astronomer Edmond Halley (who was studying his now-famous comet), Newton continued to study his notion of gravity and apply it to the motions of the Earth, sun and moon. It all led to his seminal work, published in 1687, called the "Principia" — considered by many as the greatest science book ever written. Newton's research stopped in 1679 when he had a nervous breakdown. Later, recovered, he spoke out against King James II, who wanted only Roman Catholics to be in powerful government and academic positions. When James was later driven out of England, Newton was elected to Parliament. He had a second breakdown in 1693, then retired from research. Isaac Newton died in 1727. Among his more eccentric pastimes, Newton also dabbled (or more than dabbled) in alchemy, also called chymistry, with some historians estimating that he wrote more than a million words of alchemical notes, according to curator of rare books at the Chemical Heritage Foundation, James Voelkel. And in March 2016, researchers announced they had found bought a 17th-century alchemy manuscript written by Newton .

The manuscript, which had been hidden in a private collection for decades and turned up at an auction at Bonhams, provided the recipe for "philosophic" mercury, which was considered a step in the process for concocting a mysterious substance known the philosopher's stone; this material was thought to have supernatural powers — the ability to turn any metal into gold and to grant immortality. The manuscript will be available online for enthusiasts to explore. Further reading Universal Gravitation – The Physics Hypertextbook Discussion the comet Isaac Newton was born on Christmas Day, 1642 in the village of Woolsthorpe (near Grantham), Lincolnshire, England. In 1661 he enrolled in Trinity College, Cambridge University (about midway between Woolsthorpe and London) where he studied mathematics. In 1665 the Black Plague made it's way to England forcing the closure of Trinity and sending Newton home to Woolsthorpe for a year or two. It was during this time that he formulated most of his important contributions to mathematics and physics including the binomial theorem, differential calculus, vector addition, the laws of motion, centripetal acceleration, optics, and universal gravitation.

Upon returning to Cambridge, Newton was made a professor of mathematics and then proceeded to do what professors still do to this day — teach and publish. Most of these papers Newton submitted for publication were on optics, especially on the theory of colors. Then, eighteen years later in 1684, Edmond Halley came to Newton with a problem he thought Newton might be able to solve. Comets are astronomical objects that are visible to the unaided for only a month or so. They were a serious problem for early astronomers as they would appear without warning, hang around in the sky for a while, and then disappear never to be seen again. Halley was studying historical records of cometary appearances when he noticed four comets with nearly the same orbit separated in time by approximately 76 years. He reasoned that the comets of 1456, 1531, 1607, and 1682 were sightings of a single comet and that this comet would reappear in the winter of 1758. When it did as predicted, sixteen years after his death, it became known as Comet Halley.

It should be noted that Halley did not discover the comet that bears his name, he was just the one who identified it as a celestial body with a definite period in orbit around the sun. Halley's comet has probably been seen since the dawn of civilization when humans first looked up and they sky and wondered how it all worked. Historical records from India, China, and Japan record its appearance as far back as 240 BCE (with one appearance not recorded). Its most recent appearances were in 1833, 1909, and 1985 and its next will be in 2061. Halley also noticed that the comet described an orbit around the sun that was in accordance with Kepler's laws of planetary motion; namely, that the orbit was an ellipse (albeit a highly elongated one) with the sun at one focus and that it obeyed the harmonic law (r3 ∝ T2) as if it was another planet in our solar system. Halley asked Newton in 1684 if he had some idea why the planets and this comet obeyed Kepler's laws; that is, if he knew the nature of the force responsible.

Newton replied that he had indeed solved this problem and "much other matter" pertaining to mechanics eighteen years earlier but hadn't told anyone about it. He then proceeded to rummage around looking for his notes from the plague years, but could not find them. Halley persuaded Newton to compile everything he ever knew on mechanics and offered to pay the costs so that his ideas might be published. In 1687, after eighteen months of effectively non stop work, Newton published Philosophiæ Naturalis Principia Mathematica (The Mathematical Principles of Natural Philosophy). Probably the single most important book in physics and possibly the greatest book in all of science, it is almost always just known as the Principia. It contains the essence of the concepts presented in the chapters on mechanics in every subsequent physics textbook, including this one. Probably the only important concept it misses is energy, but everything else is there: force, mass, acceleration, inertia, momentum, weight, vector addition, projectile motion, circular motion, satellite motion, gravitation, tidal forces, the precession of the equinoxes….

… In 1684 Dr Halley came to visit him at Cambridge, after they had been some time together, the Dr asked him what he thought the curve would be that would be described by the Planets supposing the force of attraction towards the Sun to be reciprocal to the square of their distance from it. Sr Isaac replied immediately that it would be an Ellipsis, the Doctor struck with joy & amazement asked him how he knew it, why saith he I have calculated it, whereupon Dr Halley asked him for his calculation without any farther delay, Sr Isaac looked among his papers but could not find it, but he promised him to renew it; & then to send it him Abraham de Moivre, 1727 … De motu corporum in gyrum (On the motion of bodies in orbit) is the (presumed) title of a manuscript by Isaac Newton sent to Edmond Halley in November 1684. the law The Principia contains in it the unification of terrestrial and celestial gravitation. The acceleration due to gravity described by Galileo and the laws of planetary motion observed by Kepler are different aspects of the same thing.

There is no terrestrial gravitation for earth and no celestial gravitation for the planets, but rather a universal gravitation for everything. Every object in the universe attracts every other object in the universe with a gravitational force. The magnitude of the gravitational force between two objects is… directly proportional to the product of their masses and inversely proportional to the square of the separation between their centers Newton's law works since we live in a universe with three spatial dimension. As gravity extends out into space it spreads itself thinner and thinner, covering an area that expands as the square of the distance from the source. If space wasn't three-dimensional, Newton's law wouldn't work. Although space appears three dimensional, there's no obvious reason why it has to be. Some advanced theories seem to suggest that there may be additional spatial dimensions. The reason we haven't seen them is that they're curled up rather tightly. If they exist, it should be possible to find deviations in the force of gravity from Newton's inverse square law at extremely small distances. Testing for these deviations is quite difficult. The best experiments currently (2001) show that the inverse square law holds down to 218 μm (2.18 × 10−4 m).

Since the size of these hidden dimensions is thought to be on the order of 10−35 m, we've still got a long ways to go. the moon The earth-moon separation is approximately sixty times greater than the radius of the Earth. The acceleration due to gravity at this distance is 1/3600 the acceleration due to gravity at the surface of the Earth. the apple Isaac Newton entered Trinity College at the University of Cambridge in 1661. He received his bachelor of arts degree in 1665 as the Great Plague was sweeping through London. The University of Cambridge closed as a precaution and Newton fled to his family's farm in Lincolnshire 90 km (60 miles) to the north. In the summer of 1666, Newton began work on his theory of universal gravitation. A bit more than twenty years later, the final theory was released to the public as a part of his grand tome Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Apples were not a part of the discussion. Jump ahead to 1726. Sir Isaac Newton was a legend nearing the end of his life.

He had dinner with a friend, William Stukeley, and they sat in a garden afterwards and talked about many things. Newton was 83 years old at the time when he recalled an event that took place 60 years earlier. This is the story as Stukeley tells it (using his original spelling, capitalization, and punctuation). on 15 April 1726 I paid a visit to Sir Isaac, at his lodgings in Orbels buildings, Kensington: din'd with him… after dinner, the weather being warm, we went into the garden, & drank thea under the shade of some appletrees, only he, & myself. amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. "why should that apple always descend perpendicularly to the ground," thought he to him self: occasion'd by the fall of an apple, as he sat in a contemplative mood: "why should it not go sideways, or upwards? but constantly to the Earths centre? assuredly, the reason is, that the Earth draws it. there must be a drawing power in matter.

& the sum of the drawing power in the matter of the Earth must be in the Earths center, not in any side of the Earth. therefore dos this apple fall perpendicularly, or toward the center. if matter thus draws matter; it must be in proportion of its quantity. therefore the apple draws the Earth, as well as the Earth draws the apple." William Stukeley, 1752 Another variation of the apple story was recorded by Newton's assistant at the Royal Mint (and also his nephew-in-law), John Conduitt. In the year [1666] he retired again from Cambridge on acct of the plague to his mother [in] Lincolnshire & whilst he was musing in a garden it came into his thought that the same power of gravity (wch made an apple fall from the tree to the ground) was not limited to a certain distance from the Earth but that this power must extend much farther than was usually thought — Why not as high as the Moon said he to himself & if so that must influence her motion & perhaps retain her in her orbit, whereupon he fell a calculating… & found it perfectly agreable to his Theory — John Conduitt, ca.

1728 Newton himself never wrote anything about apples. He was more interested in the motion of the moon as a means to test his theory. In the same year [1666] I began to think of gravity extending to the orb of the moon, and having found out how to estimate the force with which a globe revolving within a sphere presses the surface of the sphere, from Kepler's rule of the periodical times of the planets being in a sesquilaterate proportion of their distances from the centres of their orbs I deduced that the forces which keep the Planets in their orbs must be reciprocally as the squares of their distances from the centres about which they revolve: and thereby compared the force requisite to keep the moon in her orb with the force of gravity at the surface of the Earth, and found them to answer pretty nearly. All this was in the two plague years of 1665 and 1666, for in those days I was in the prime of my age for invention, and minded mathematics and philosophy more than at any time since. Isaac Newton, ca.

1715 When Newton was asked how he discovered the law of universal gravitation, his reply was… [I]f I have done ye publick any service this way 'tis due to nothing but industry & a patient thought. the constant Cavendish experiment The Great Pyramid is so massive that a plumb line will not hang straight down when near the pyramid but will swing toward the structure. Cf. Tompkins, Secrets of the Great Pyramids, pp. 84-85, where Tompkins, discussing the measurements taken by Piazzi Smyth, writes "To obtain the correct latitude of the Great Pyramid without having his plumb line diverted from the perpendicular by the attraction of the huge bulk of the Pyramid, Smyth made his observations from the very summit; there the Pyramid's pull of gravity would be directly downward". Tompkins, Peter. Secrets of the Great Pyramid (New York: Harper Collins, 1971). the critics Action at a distance. Newton's reply to these criticisms was basically, "I don't care. The theory works." Rationem vero harum gravitatis proprietatum ex phænomenis nondum potui deducere, & hypotheses non fingo….

Et satis est quod gravitas revera existat, & agat secundum leges a nobis expositas, & ad corporum cælestium & maris nostri motus omnes sufficiat. I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypotheses…. And to us it is enough that gravity does really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies, and of our seas. beyond that… Somebody invented the gravitational field. Units: N/kg or m/s2 Happy equivalence of inertial and gravitational mass. No doubt, Newton thought God spoke to him, but the Bible does not mention the law of universal gravitation. Newton went insane for a couple of years, probably due to mercury poisoning. Newton thought that his greatest accomplishment was that he died a virgin. He thought more of his biblical analysis than his physical analysis. Newton was appointed master of the mint — basically a patronage position to reward him for his accomplishments in physics.

While there he implement serrated coins in an effort to prevent coin "clipping" or "shaving", which was a serious problem in England at the time. Newton coined the word gravity from gravitas, the Latin word for heaviness, severity, or authority. The Latin word for weight is pondus, which gives us the English unit of weight — the pound (but interstingly, not the verb to pound or the pound where stray animals are kept). MAKE A NEW SECTION FOR GRAVITATIONAL FIELD extra dimensions A (modified) quote from The Physics Teacher that will be paraphrased. "An essential ingredient for black hole production at the LHC (Large Hadron Collider) is the existence of extra dimensions. A black hole is a region of intense gravitational field creating conditions that are contrary to what we observe about gravitational forces to be in our everyday world. The presence of extra dimensions guarantees the extra strength of gravity needed to produce black holes. When protons collide at the LHC, they come so close to each other that they essentially "see" the extra dimensions (where gravity is strong) and black hole formation may be possible. If this is the case, then the extra dimensions must be ~10−14 m in size.

" Three dimensional space

Question: Who discovered the law of gravity?
Answer: Sir Isaac Newton